BACKGROUND: The goal of this study was to examine the association between low levels of lead and mercury in blood and symptoms of attention-deficit hyperactivity disorder (ADHD) among Korean children. METHODS: One thousand seven hundred and seventy eight children at 10 elementary schools in six South Korea cities participated in this study. Parents and guardians administered a questionnaire including Conners' parents rating ADHD scale to determine the presence of ADHD symptoms. In addition, clinical examinations of the children and determination of blood lead and mercury levels were included in the first Children's Health and Environment Research (CHEER) survey, which is now conducted annually in Korea. RESULTS: The risk for the appearance of ADHD symptoms was found to increase with the blood lead concentration. The mean blood lead concentration was low with a geometric mean of 1.8 microg/dl. The odds ratios (95% confidence intervals) for the presence of ADHD symptoms were 1.28 (0.57, 2.86), 1.32 (0.63, 2.74), 1.65 (0.77, 3.56), and 1.98 (0.76, 5.13) in children with blood lead levels of 1-<1.5, -<2.5, -<3.5, and >3.5 microg/dl, compared to those with blood lead levels of <1.0 microg/dl; these results statistically represented a borderline trend (p for trend: 0.07). The blood lead level showed a significant positive association with the Conners' ADHD score (beta=0.50, p<0.0001). However, the blood mercury levels were not found to be significantly associated with ADHD symptoms in children. The geometric mean mercury concentration in the blood was 2.4 microg/l. CONCLUSIONS: The observed association between blood lead concentration and the appearance of ADHD symptoms in Korean children suggests that lead, even at low concentrations, is a risk factor for ADHD.